1. Field of the Invention
The invention concerns a photoresist composition for UV irradiation with a wavelength below 200 nm, and a process for the lithographic treatment of a substrate.
2. Brief Description of Prior Art
In the case of the microlithographic production of structures on a substrate, as is known, the substrate, e.g., silicon, silicon oxide or silicon nitride is coated with a photoresist composition and this coating is selectively irradiated with radiation of a suitable wavelength, so that the irradiated resist material has a different solubility than the unexposed material in a developer solution. The irradiated coating is finally treated with the developer, whereby it is structured as an image on the basis of the different solubilities. The imaged coated substrate can then be subjected to a further treatment on those places free of the resist. Frequently, this involves a plasma etching (e.g., a silicon oxide or silicon nitride substrate is etched with a halogen/oxygen plasma mixture, e.g., a Cl.sub.2 -O.sub.2 or a CF.sub.4 -F.sub.2 plasma) against which the resist coating must be sufficiently stable. The microlithographically maximally obtainable resolution is determined essentially by the radiation wavelengths used for the selective irradiation. For some time now, the use of deep UV radiation has been employed for many applications, particularly the application of radiation with a wavelength of 200-250 nm. For example, radiation of 248 nm is broadly utilized and this can be produced with krypton fluoride excimer lasers.
The resolution capacity that can be obtained with conventional deep UV microlithography, however, has its limits. It permits structures with dimensions down to approximately 0.20 .mu.m to be resolved. However, the targeted production most recently of 4 gigabyte storage elements, for example, or of similarly highly-integrated electronic components requires the formation of essentially smaller structural elements, which have minimal dimensions of down to approximately 0.12 .mu.m. In order to be able to sufficiently resolve optically such small structural elements, wavelengths shorter than deep UV radiation must be utilized. In particular, the radiation of argon fluoride excimer lasers, which has a wavelength of 193 nm is considered for this purpose. The deep UV photoresist material that can be used today, however, is not suitable for 193 nm radiation. That current material is based usually on phenolic resins as a binding agent, particularly on novolak resins or on polyhydroxystyrene derivatives, which have too high an absorption at wavelengths below 200 nm, due to their aromatic structural elements. This high absorption leads to the result with 193 nm radiation that the side walls of the finished developed resist structures do not form the desired vertical profiles. But instead a more or less oblique angle with the substrate or with the resist surface is obtained which causes poor optical resolution characteristics at the application of these short-wave radiation.
It has thus been proposed to utilize photoresists based on methacrylate resins for the production of structures by means of 193-nm radiation (See Roderick R. Kunz et al. "RESIST PROCESS FOR ArF EXCIMER LASER LITHOGRAPHY", Journal of Photopolymer Science and Technology, Vol. 6 (1993), pp. 473-490). Such photoresists have in fact proven sufficiently transparent for this radiation, but they do not have the etching stability customary for resists based on phenolic resins in the case of plasma etching. That etching stability is based essentially on the aromatic groups in the novolak-containing resists (See James R. Sheats: "PHOTORESISTS FOR DEEP UV LITHOGRAPHY" SOLID STATE TECHNOLOGY, 1989, pp. 79-86).
There have also been several attempts to reduce this problem. For example, it has been attempted to improve the etching stability of photoresists based on meth(acrylate) by introducing cycloaliphatic groups into the meth(acrylate) polymers (See Robert D. Allen et al. "SINGLE LASER RESISTS WITH ENHANCED ETCH RESISTANCE FOR 193 nm LITHOGRAPHY", Journal of Photopolymer Science and Technology, Vol. 7 (1994), pp. 507-516). This in fact leads to an improvement in etching stability, but not to the desired extent. Thus, for example, the etching rate of poly(isobornyl)acrylate in oxygen or chlorine plasma is still approximately 50-70% higher than that of novolak resins (See Roderick R. Kunz et al. "RESIST PROCESS FOR ArF EXCIMER LASER LITHOGRAPHY", Journal of Photopolymer Science and Technology, Vol. 6 (1993), pp. 473-490).
Another proposal aims at producing a sufficient etching stability only after irradiation in the resist coating. For this purpose, it has been proposed to treat the substrate with the finished, developed, image-structured photoresist coating in a suitable way with specific alkyl compounds of magnesium or aluminum, in order to introduce the given metals in the resist material in this way as etching barriers (See U.S. Pat. No. 4,690,838). The use of metal-containing reagents, however, is generally not desired in the case of the microlithography process, due to the danger associated therewith of a contamination of the substrate with metal ions.
In addition, nonaromatic chemical compounds which can be converted in a relatively simple way into compounds with aromatic groups (designated below also as "latent aromatic compounds"), are known. Bicycle[3.2.2]nona-6,8-dien-[3]-one reacts this way under acid catalysis to form phenyl acetone (See A. E. Hill et al. "ONE STAGE SYNTHESIS OF BICYCLO[3.2.2]NONA-6,8-DIEN-3-ONES", Journal of the American Chemical Society, Vol. 96 (1974), pp. 4597-4603) as do also specific poly(cyclohexadiene carbonates) and acetates also under acid catalysis or also only upon heating to form polyphenylenes (See U.S. Pat. No. 5,248,734).
A chemically reinforced positive photoresist for application in the deep UV region, which comprises poly(cyclohexa-1,3-dien-5,6-diacetate) and triphenylsulfonium trifluorosulfonate as radiation-sensitive acid donors, has also been described (See Japanese Published Patent Application No. 05/061198). After exposure with 248 nm radiation and removal of the irradiated parts of the resist layer with an alkaline developer solution, the substrate is heated with the image-structured resist coating according to the instructions of this document to 200.degree. C., so that the poly(cyclohexa-1,3-diene-5,6-diacetate) reacts in the remaining part of the coating to form polyphenylene. The named photoresist shows a good transparency for the radiation of 248 nm wavelength and a good stability with a later plasma etching.
It has now been found that by means of latent aromatic compounds, photoresist compositions can be formulated, which produce a resist coating, which is still sufficiently transparent for radiation with a wavelength of approximately 193 nm and which, however, is sufficiently stable in plasma etching after conversion of the latent aromatic groups to aromatic groups. This was unexpected, particularly since the known latent aromatic groups contain isolated carbon double bonds, which, as is known, show a strong absorption band at approximately 190-200 mm. In this way, it is possible for the first time to formulate a wet-chemical, developable photoresist for radiation of approximately 193 nm wavelength, which shows an etching rate that is comparable to conventional resists based on phenolic resin, and this is true without needing to treat the resist coating with metal compounds in order to increase the etching stability.